It is both desirable and necessary to correct signal distortion in high-speed optical communications over optical fibers.
In an optical fiber, ultra-short pulses of light carry the signal. Initially, in each pulse, the electric field of the light follows a given direction. Then, because the optical fiber is not perfectly circular, the direction of the electric field, or polarization, splits into two components that propagate at different speeds, causing the pulse to spread, an effect referred to as polarization mode dispersion (PMD).
External fluctuations in the ambient conditions such as temperature, mechanical vibration, etc. cause the PMD to vary with time. At very high transmission rates, which can reach beyond 40 Gb/s, these time varying distortions are so severe that they need to be compensated for to achieve reliable operation of the optical signal transmission. Current optical transmission systems include, at regular intervals, PMD-compensating devices, which incorporate a device, known as a polarization controller, to control the polarization state of the optical pulses.
Presently, lithium niobate (LiNbO3) based polarization controllers (PolCons) can adjust the polarization fast enough to adequately compensate for the time variations in the PMD. Unfortunately, such lithium niobate devices are very expensive.
Polarization controllers (PolCons) based on variable-birefringence nematic liquid crystal devices known in the art have finite tuning range and hence require complicated resetting algorithms. Other prior known devices based on rotatable nematic liquid crystal wave plates avoid this problem, but require large numerical aperture bulk optics for controlling and focusing the light, and they have relatively slow, millisecond switching speeds.